Glass-bonded ferrite



Jan. 21, 1958 :A. H. IVERSEN 2,

GLASS-BONDED FERRITE Filed Sept. 15, 1954 j s Sheets-Sheet 1 Wwwrm 1 ,lrmm zn Jan. 21, 1958 A. H- IVERSEN GLASS-BONDED FERRITE 3 Sheets-Sheet 2 Filed Sept. 15, 1954 Zia [Mae 40/! 4 Wax/4 Jan. 21, 1958 A. H.'IVERSEN I 2,820,720

GLASS-BONDED FERRITE Filed Sept. 15, 1954 5 Sheets-Sheet 3 Away/44.

United States Patent" 2,820,720 Patented Jan. 21, 1958.

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ferrite body'and .preve'nts 'the moltenxglass 'fromssoakingtin. The amount and thickness-of the glass needed to-'efiecti'vely seal the ferrite body and therefore th'eiloss of t the r composite structure is thereby reduced.

The novel features which are believed to he -characteristic of -the-invention,-'--bo'th as :to' itsorganization-and methodtofloperation; together withrfurther objects and advantages thereofyawill be better-understood from the following description considered in connection with the accompanyingidrawingsain which: an embodiment of the inventionmis illustrated r'byiway .of exarnple. It is to be expresslyunderstoodghowever, that the drawings are-fer the 'purposeof illustration-and description only,and are I notrintended as az definltiontof the"limits of the inven This t invention relates to vitreous-coatedceramics and more particularly to a glass bond for porousmagnetic" materials and a method of making such'a bond.

At microwave frequencies ferromagnetic ceramic structurestcommonly called ferrites are employedeto -advantage in several different ways, one of which is de-- scribed and-claimed in a copending application vSerial NOL 419,259: by Willard'A. Hughes, filed March 29, 1954;;entitled- :Waveguide Isolator. Ferritesl generally consistof asmixture of-the oxides of various bivalent metals with ferric oxide 'Fe O The general formula" of.atferriteis Me ':Me :Ee O where Me may be a metal. such ass-nickel, copper; manganesetor'magnesi-um: and where Me maybe =a metalsuch! aszinc or cadmiu'mi A- ferrite body-may be produced by intimately mixingrE finelyedivided particles of the-metallic oxides, compactingothetrnixture by compression molding andheating it to" a temperature-of between-1,000-and =1,600Idegrees= centigrade. g

It has been found desirable to hermetically seal .the ferrite disclosed in the aforementioned Hughes applicationbecause-the presence ofmoisture-withindt: adverse- 1y: affects its performance. To this end a method of 'toatingthetferrite -with. glass has been devised which-is de-"*" scribed and claimed in copending application Seria1vNo."- 430,841. by,Arthur H. Iversen, filed: May 19, 1954; zen-n tit-led .Vitreousr- Coated-Magnetic Material,v of r whichtheinstant application isa continuation in-part;

Attmethod: of sealingJ-a porous ferritefort-preventing therescapea of moisture and occluded gases-from'a ferrite' body in a high-vacum electron tube was'disclosed' in the 4 Iv'ersen application. This methodernploystheuse of two-glasses having substantially different sin-teringitern peraturesw The glasszhaving the'high sinte-ringtempera ture. islsintered on the ferrite body first to fill totally or partially the largest of the ferrite pores. The second glass -is.then sintered over the first. This method "is veryuseful in -hermetically sealing porous ferrite bodies for usein Vacuum tubes. I The method is also usefulwin sealingtthe ferritebody disclosed in the Hughes applica-"' tion; however, in that :applicationthe use of a single=thinf layer of a low-loss glass is oftendesirable."

An objectof the invention is therefore to provide a bond between a porous ferrite and a thin glass coating having a low-loss and a method of providing such a bondi In accordance with the present invention a smallamount ofdielectric or ferrite powder is applied tofthe" outer surface of a ferrite body and sintered there. After-1 the ferrite. body ,isvallowed to cool .a suitable-glass is flowed overtthe sintered dielectric surface;- The dielec'- trip powder, when sintered, closes the-largeporesmf the tiOIL' Figaro-1 is anelevational viewgpartly in section; of a glass bonded ferrite; a

Fig. ,2 ista'sectional view on thevline 2-2of- Fig. .1;

Fig: 3 illustrates oneistep in the improved methodrof coatingiia porous-ferrite body with glass;

Fig. 4 shows: another step in the'rnethodof bonding glass toa ferrite body;

Figs-'5, 6 and"7 arephotographs oftheferrite body in---.vari0us stages of manufacture;-

Fig. -8-is--a =pho'tomierograph of a ferrite 'body illus-r tratingzhow glass soaks int'o-atporous-ferriteyand 1 Fig.9 is =a photomicrograph of the glass to iferrite'f' bond capa'blev of beingproduced in accordance with the present-invention;v

ferrite body-=10; whiehds encasedoby a glass coating lzy has:a dense:interface layer of a, dielectric material 14;

The dielectric material may beconstitutedvof a sintered layer offia finely grou'rid or ball milledopowderwf-the ferrite material itself-. having a particle size equalto, for

exampl'e,-"five rnicronsr: The ferrite body-"10,'glass coat ingi12-and dielectric :layer 14' are'likewise exhibited in the sectionalaview= of th'e'glass bonded' ferrite structure imFig. 2.

with the dielectric powder 14"as" illustrated in'Fig; 3.- Alternatively, a the "dielectric powder'14 may be suspended inwater or: alcoholand painted: or sprayed on: the ferrite J both/ 10. The dielectrid powder 14 is then 'sintered at iv a temperature less-thanthe sinter'ing temperature-of the ferrite body 10,-say-l0O0- degrees-'centigrad'er This is done to prevent grain growtlrin'the ferrite body 10 which may produce an undesirable change in its magnetidproperties. Aphotograph of the ferrite body 10 with-a sintered dielectric surface is shown in Fig.- 6. The

dielectric powder 14' then forms a relatively non-porous interface layer over the ferrite body 10. The glass ;12, in a powdered form is suspended in a suitable fluid medium such as water or alcohol and the suspension isapplied to the non-porous sintered dielectric surface 14" of the ferritebo'dy" 10 by brush 16 'as shown in Fig. 4.1 The ferrite body 10 may, of course, alternatively be" sprayed withthe powdered glass suspension. The glass M -itself is'then 'sintered about thedielectric layer 14' the ferrite body 10 at a temperature less than the sintering temperature of the dielectric powder 14, e. g.'at 700 degrees centigrade. The glass-bonded ferrite body is thus shown in the photograph of Fig. 7.

The fact that glass will soak into a porous ferrite is shown in the photomicrograph of Fig. 8 where a porous ferrite body 80 has been coated with a glass 82 that has soaked into the ferrite body 80 at the layer 84. However, with the present invention theglass 12 is prevented from soaking into the ferrite body 10. This is clearly illustrated by the photomicrograph of Fig. 9 showing an enlarged sectional view of the ferrite body 10, the glass 12 and the sintered dielectric surface layer 14. The darkened portions 50 and 52 of the photomicrographs of Figs. 8 and 9 are background materials.

The structure of the present invention therefore comprises a ferromagnetic ferrite body, a sintered layer of a powdered dielectric and a sealing glass cover. The particular ferrite composition is chosen for its most desirable physical properties. The glass should wet the ferrite and the thermal expansion coefficient of the glass should match the thermal expansion coefficient of the ferrite. Many suitable glasses such as lime or lead glasses which are available have been suggested in the earlier Iversen application of which the present invention is a continuation-in-part. A typical lime glass has the following weight percentages composition: SiO 70.5, Al O --l.8, Ca6.7, Na O--16.7, K O0.8, and MgO--3.4; and a typical lead glass has the following weight percentages composition: SiO 56.5, Al O -l.5, PbO--29, Ca0- 0.2, Na O5.6, K O6.6, and MgO-0.6. A glass designated 8870 by the Corning Glass Works has an unusually high thermal expansion coefiicient with an unusually low-loss factor. Since low-loss is the prime consideration in the present invention, it is obviously desirable to choose a glass having a low-loss factor and to flow the glass onto the ferrite with a minimum thickness. Heating the ferrite body 10 to an appropriate temperature may further serve to evaporate the glass suspending medium. A glass designated 7570 by the Corning Glass Works and sometimes called solder glass is a glass which exhibits certain satisfactory physical prop-- erties for sealing a ferrite in that it is desirable to use a. glass having a sintering temperature below that of the dielectric powder 14, which Corning glass 7570 has.

Having chosen a particular ferrite composition, such as that used in ferrite bodies distributed under the trade name Ferroxcube 106 by the Ferroxcube Corporation, and a glass composition, such as Corning glass 7570, the only variable which remains to be determined in the manufacture of the glass bonded ferrite is the particle size of the ferrite powder. The particle size is the only variable because the dielectric layer 14 is generally so small that as long as the ferrite powder or dielectric material 14 has a low or equal loss and is non-conductive, its presence has little, if any, effect upon the magnetic properties of the ferrite body 10. The interface layer 14 may therefore be constituted of practically any dielectric material including the material of the ferrite body itself. The particle size, e. g. microns, of the powdered dielectric material 14 should therefore be reduced to have a sintering time, e. g. minutes, and temperature, e. g. 1000 degrees centigrade to additionally drive off occluded gases from the ferrite body 10 substantially below that which may produce a change in the original magnetic properties of the ferrite body itself. Sintering may also serve to evaporate the dielectric powder suspending medium; For this purpose, the sintering temperature may be defined as that temperature at which substantial fusion takes place. defined as the time of heating required to sinter the dielectric powder.

The method that follows is a typical process of the present invention which may be emp'loyed to seal a Sintering time may be porous ferrite without substantially increasing its loss:

('1) Apply a ferrite powder having a 5 microns particle size to a porous ferrite body by painting or spraying a suspension of the powder in water onto the external surface of the ferrite body as shown in Fig. 3.

(2) Sinter the ferrite powder and body in air at 1000 degrees centigrade for ten minutes.

(3) After allowing the ferrite body to cool, paint or spray a liquid suspension of Corning glass 7570 in water onto the sintered layer of the ferrite powder as illustrated in Fig. 4 using eight parts water to one part glass by volume.

(4) Flow the Corning glass 7570 by heating it to its sintering temperature, which may be between 550 to 750 degrees centigrade, for about ten minutes.

Alternatively methyl alcohol may be employed as the glass or ferrite powder suspending medium.

What is claimed is:

1. A glass-bonded ferrite structure comprising a ferrite body, a sintered substantially non-porous layer of a dielectric ferrite powder being less than 5 microns in particle size and disposed on the external surface of said ferrite body, and a soft glass fused over said sintered layer of ferrite powder.

2. A glass-bonded ferrite structure comprising a ferrite body, a sintered substantially non-porous layer of a ferrite powder being 1 to 5 microns in particle size and disposed on the external surface of said ferrite body, and a fused cover of solder glass disposed on said sintered 9 layer.

ferrite powder to the sintering temperature of said ferrite powder, applying a liquid suspension of a powdered soft glass to the sintered surface of said ferrite powder, and heating the ferrite body, said ferrite powder and said soft glass to the sintering temperature of said soft glass.

4. The method of making a glass-bonded ferrite structure including a ferrite body, said method comprising the steps of spraying a liquid suspension of a ferrite powder onto the external surface of the ferrite body, the size of the particles in the ferrite powder being of the order of one micron such that the sintering temperature of said ferrite powder is between the sintering temperature of the ferrite body and the sintering temperature of a soft glass, whereby the physical properties of said ferrite body may be maintained constant, heating the ferrite body and the ferrite powder to the sintering temperature of the ferrite powder, brushing a liquid suspension of a powdered soft glass onto the sintered surface of the ferrite powder, and heating the ferrite body, the ferrite powder and the soft glass to the sintering temperature of the soft glass.

5. The method of making a glass-bonded ferrite strucpowder and the glass to the sintering temperature of the glass.

6. The method of making a glass-bonded fem'te structure including a ferrite body, said method comprising the steps of: applying a liquid suspension of a low loss ferrite powder onto the external surface of the ferrite body, the particles of said powder being of such a size that the sintering temperature of said powder is between the sintering temperature of the ferrite body and that of a soft glass; heating the ferrite body and the ferrite powder to the sintering temperature of said powder whereby the magnetic properties of said ferrite body are maintained substantially constant during the sintering of the ferrite powder; applying a liquid suspension of a powdered soft glass to the sintered surface of the ferrite 10 powder; and heating the ferrite body and the ferrite References Cited in the file of this patent UNITED STATES PATENTS 2,330,129 Lucas Sept. 21, 1943 2,568,881 Albert-Schoenberg Sept. 25, 1951 FOREIGN PATENTS 620,009 Great Britain Mar. 17, 1949 

1. A GLASS-BONDED FERRITE STRUCTURE COMPRISING A FERRITE BODY, A SINTERED SUBSTANTIALLY NON-POROUS LAYER OF A DIELECTRIC FERRITE POWDER BEING LESS THAN 5 MICRONS IN PARTICLE SIZE AND DISPOSED ON THE EXTERNAL SURFACE OF SAID FERRITE BODY, AND A SOFT GLASS FUSED OVER SAID SINTERED LAYER OF FERRITE POWDER. 